Like Biochemical Oxygen Demand (BOD) and Carbonaceous Biochemical Oxygen Demand (CBOD), Chemical Oxygen Demand (COD) is a test used to determine the oxygen-depleting capacity of water polluted with organic compounds. When ambient microbes break down the organic pollutants, they use up dissolved oxygen (DO) in the water that is critical for higher aquatic life forms. The COD test also looks at inorganic pollutants that can deplete oxygen as they degrade. Polluted, oxygen-depleting water released into the environment can wreak havoc downstream, where it can cause fish kills and dead stretches of water.
Like the CBOD test, COD tests only the carbonaceous sources of pollution in water; it does not measure nitrogenous pollutants. But because it also measures inorganic oxygen users, COD test results will always be higher than BOD results for the same sample. An some cases, a stable ratio can be observed between BOD and COD results at the same site. For instance, BOD may typically be 60% lower than COD results. In those cases, the EPA may allow the quicker COD test to be substituted for the 5-day-long BOD test (40 CFR 133.104 (b)).
The COD hold time is also much more forgiving, at 28 days (compared to 24 hours).
COD results are reported in mg (O2)/L.
Wikipedia, "Chemical oxygen demand"
MEL Test Method for COD
Wastewater - HACH 8000 (NPDES compliant)
Container: plastic, glass
Volume: 60 mL
Hold Time: 28 days
Preservation: H2SO4 to pH <2 , ≤6°C